Macaques are trained to press a lever when and only when electrical stimuli are applied to striate cortex. A microelectrode is then introduced which records the activity of single units in the various cytoarchitectonic laminae, and which can be used to apply highly localized stimuli. Preliminary experiments show the monkey can detect 0.2-msec pulses of as little as 2 microns A at 50 Hz, and that a several fold increase may be required if the electrode is moved as little as 100 micrometers. The experiments will first endeavor to relate these wide differences in threshold for detectability to location within the cortex and to the nature of background activity at the stimulated locus. Ultimately it is hoped to define a difference in pattern of neural discharge around a given stimulus locus when the monkey can versus when it cannot detect the applied stimuli. The experiments are thus relevant not only to the prosthetic and/or therapeutic use of central stimulation, but to questions concerning the organization of cellular activity in the "visual" cortex and the encoding of that activity into perception.